Dietary fiber (both soluble and insoluble) is a nutrient important for health, digestion, and preventing conditions such as heart disease, diabetes, obesity, diverticulitis, and constipation. However, most humans do not consume the daily recommended intake of dietary fiber. The 2010 Dietary Fiber Guidelines for Americans (U.S. Department of Agriculture and U.S. Department of Health and Human Services. Dietary Guidelines for Americans, 2010. 7th Edition, Washington, DC: U.S. Government Printing Office, December 2010) reports that the insufficiency of dietary fiber intake is a public health concern for both adults and children. As such, there remains a need to increase the amount of daily dietary fiber intake, especially soluble dietary fiber suitable for use in a variety of food applications.
Historically, dietary fiber was defined as the non-digestible carbohydrates and lignin that are intrinsic and intact in plants. This definition has been expanded to include carbohydrate polymers that are not significantly hydrolyzed by the endogenous enzymes in the upper gastrointestinal tract of humans, and additionally are not significantly fermented by the microbiota present in the lower gastrointestinal tract. Soluble oligosaccharide fiber products (such as oligomers of fructans, glucans, etc.) are currently used in a variety of food applications. However, many of the commercially available soluble fibers have undesirable properties such as low tolerance (causing undesirable effects such as abdominal bloating or gas, diarrhea, etc.), lack of digestion resistance, high cost or a production process that requires at least one acid-catalyzed heat treatment step to randomly rearrange the more-digestible glycosidic bonds (for example, α-(1,4) linkages in glucans) into more highly-branched compounds with linkages that are more digestion-resistant. A process that uses only naturally occurring enzymes to synthesize suitable glucan fibers from a safe and readily-available substrate, such as sucrose, may be more attractive to consumers.
Various bacterial species have the ability to synthesize dextran oligomers from sucrose. Jeanes et al. (JACS(1954) 76:5041-5052) describe dextrans produced from 96 strains of bacteria. The dextrans were reported to contain a significant percentage (50-97%) of α-(1,6) glycosidic linkages with varying amounts of α-(1,3) and α-(1,4) glycosidic linkages. The enzymes present (both number and type) within the individual strains were not reported, and the dextran profiles in certain strains exhibited variability, where the dextrans produced by each bacterial species may be the product of more than one enzyme produced by each bacterial species.
Glucosyltransferases (glucansucrases; GTFs) belonging to glucoside hydrolase family 70 are able to polymerize the D-glucosyl units of sucrose to form homooligosaccharides or homopolysaccharides. Glucansucrases are further classified by the type of saccharide oligomer formed. For example, dextransucrases are those that produce saccharide oligomers with predominantly α-(1,6) glycosidic linkages (“dextrans”), and mutansucrases are those that tend to produce insoluble saccharide oligomers with a backbone rich in α-(1,3) glycosidic linkages. Mutansucrases are characterized by common amino acids. For example, A. Shimamura et al. (J. Bacteriology, (1994) 176:4845-4850) investigated the structure-function relationship of GTFs from Streptococcus mutans GS5, and identified several amino acid positions which influence the nature of the glucan product synthesized by GTFs where changes in the relative amounts of α-(1,3)- and α-(1,6)-anomeric linkages were produced. Reuteransucrases tend to produce saccharide oligomers rich in α-(1,4), α-(1,6), and α-(1,4,6) glycosidic linkages, and alternansucrases are those that tend to produce saccharide oligomers with a linear backbone comprised of alternating α-(1,3) and α-(1,6) glycosidic linkages. Some of these enzymes are capable of introducing other glycosidic linkages, often as branch points, to varying degrees. V. Monchois et al. (FEMS Microbiol Rev., (1999) 23:131-151) discusses the proposed mechanism of action and structure-function relationships for several glucansucrases. H. Leemhuis et al. (J. Biotechnol., (2013) 163:250-272) describe characteristic three-dimensional structures, reactions, mechanisms, and α-glucan analyses of glucansucrases.
A non-limiting list of patents and published patent applications describing the use of glucansucrases (wild type, truncated or variants thereof) to produce saccharide oligomers has been reported for dextran (U.S. Pat. Nos. 4,649,058 and 7,897,373; and U.S. Patent Appl. Pub. No. 2011-0178289A1), reuteran (U.S. Patent Application Publication No. 2009-0297663A1 and U.S. Pat. No. 6,867,026), alternan and/or maltoalternan oligomers (“MAOs”) (U.S. Pat. Nos. 7,402,420 and 7,524,645; U.S. Patent Appl. Pub. No. 2010-0122378A1; and European Patent EP1151085B1), α-(1,2) branched dextrans (U.S. Pat. No. 7,439,049), and a mixed-linkage saccharide oligomer (lacking an alternan-like backbone) comprising a mix of α-(1,3), α-(1,6), and α-(1,3,6) linkages (U.S. Patent Appl. Pub. No. 2005-0059633A1). U.S. Patent Appl. Pub. No. 2009-0300798A1 to Kol-Jakon et al. discloses genetically modified plant cells expressing a mutansucrase to produce modified starch.
Enzymatic production of isomaltose, isomaltooligosaccharides, and dextran using a combination of a glucosyltransferase and an α-glucanohydrolase has been reported. U.S. Pat. No. 2,776,925 describes a method for enzymatic production of dextran of intermediate molecular weight comprising the concomitant action of dextransucrase and dextranase. U.S. Pat. No. 4,861,381A describes a method to enzymatically produce a composition comprising 39-80% isomaltose using a combination of a dextransucrase and a dextranase. Goulas et al. (Enz. Microb. Tech (2004) 35:327-338 describes batch synthesis of isomaltooligosaccharides (IMOs) from sucrose using a dextransucrase and a dextranase. U.S. Pat. No. 8,192,956 discloses a method to enzymatically produce isomaltooligosaccharides (IMOs) and low molecular weight dextran for clinical use using a recombinantly expressed hybrid gene comprising a gene encoding an α-glucanase and a gene encoding dextransucrase fused together; wherein the glucanase gene is a gene from Arthrobacter sp., wherein the dextransucrase gene is a gene from Leuconostoc sp.
Hayacibara et al. (Carb. Res. (2004) 339:2127-2137) describes the influence of mutanase and dextranase on the production and structure of glucans formed by glucosyltransferases from sucrose within dental plaque. The reported purpose of the study was to evaluate the production and the structure of glucans synthesized by GTFs in the presence of mutanase and dextranase, alone or in combination, in an attempt to elucidate some of the interactions that may occur during the formation of dental plaque.
Mutanases (glucan endo-1,3-α-glucanohydrolases) are produced by some fungi, including Trichoderma, Aspergillus, Penicillium, and Cladosporium, and by some bacteria, including Streptomyces, Flavobacterium, Bacteroides, Bacillus, and Paenibacillus. W. Suyotha et al., (Biosci, Biotechnol. Biochem., (2013) 77:639-647) describe the domain structure and impact of domain deletions on the activity of an α-1,3-glucanohydrolases from Bacillus circulans KA-304. Y. Hakamada et al. (Biochimie, (2008) 90:525-533) describe the domain structure analysis of several mutanases, and a phylogenetic tree for mutanases is presented. I. Shimotsuura et al, (Appl. Environ. Microbiol., (2008) 74:2759-2765) report the biochemical and molecular characterization of mutanase from Paenibacillus sp. Strain RM1, where the N-terminal domain had strong mutan-binding activity but no mutanase activity, whereas the C-terminal domain was responsible for mutanase activity but had mutan-binding activity significantly lower than that of the intact protein. C. C. Fuglsang et al. (J. Biol. Chem., (2000) 275:2009-2018) describe the biochemical analysis of recombinant fungal mutanases (endoglucanases), where the fungal mutanases are comprised of a NH2-terminal catalytic domain and a putative COOH-terminal polysaccharide binding domain.
Glucans comprising α-(1,6) glycosidic linkages can be enzymatically produced from maltodextrin. The enzyme dextrin dextranase (“DDase”; E.C. 2.4.1.2; sometimes referred to in the alternative as “dextran dextrinase”) from Gluconobacter oxydans has been reported to synthesize dextrans from maltodextrin substrates. DDase catalyzes the transfer of the non-reducing terminal glucosyl residue of an α-(1,4) linked donor substrate (i.e., maltodextrin) to the non-reducing terminal of a growing α-(1,6) acceptor molecule. Naessans et al. (J. Ind. Microbiol. Biotechnol. (2005) 32:323-334) reviews a dextrin dextranase and dextran from Gluconobacter oxydans. 
Others have studied the properties of dextrin dextranases. Kimura et al. (JP2007181452(A)) and Tsusaki et al. (WO2006/054474) both disclose a dextrin dextransase. Mao et al. (Appl. Biochem. Biotechnol. (2012) 168:1256-1264) discloses a dextrin dextranase from Gluconobacter oxydans DSM-2003. Mountzouris et al. (J. Appl. Microbiol. (1999) 87:546-556) discloses a study of dextran production from maltodextrin by cell suspensions of Gluconobacter oxydans NCIB 4943.
JP4473402B2 and JP2001258589 to Okada et al. disclose a method to produce dextran using a dextrin dextranase from G. oxydans in combination with an α-glucosidase. The selected α-glucosidase was used hydrolyze maltose, which was reported to be inhibitory towards dextran synthesis.
An “GtfB-type” α-glucosyltransferase that uses α-(1,4) linked glucooligosaccharides substrates instead of sucrose to produce glucooligosaccharides having α-(1,6) glycosidic linkages has also been described. U.S. Patent App. Pub. No. 2012-0165290 to Dijkhuizen et al. describes an GtfB α-glucosyltransferase from Lactobacillus reuteri and its use in a method for producing a mixture of glucooligosaccharides having one or more α-(1,6) glucosidic linkages and one or more consecutive α-(1,4) glucosidic linkages by contacting a poly- and/or oligosaccharide substrate comprising at least two α-(1,4) linked D-glucose units with the GtfB under suitable reaction conditions.
The enzymatic addition of α-(1,2) branching to dextrans has been reported. A glucosyltransferase (DsrE) from Leuconostoc mesenteroides NRRL B-1299 has a 2nd catalytic domain (“CD2”) capable of adding α-(1,2) branching to dextrans (U.S. Pat. Nos. 7,439,049 and 5,141,858; Published U.S. Patent Appl. Pub. No. 2009-0123448; and Bozonette et al., J. Bacteriol. (2002) 184(20):5723-573). U.S. Patent Appl. Pub. No. 2010-0284972 describes methods and compositions for improving the health of a subject by administering compositions comprising α-(1,2) branched α-(1,6) dextrans. Sarbini et al. (Appl. Envion. Microbiol. (2011) 77(15):5307-5315) describes in vitro fermentation of dextran and α-(1,2) branched dextrans by the human fecal microbiota. Brison et al. (J. Biol. Chem., (2012) 287(11):7915-7924) describes a truncated form of the DsrE glucosyltransferase from Leuconostoc mesenteroides NRRL B-1299 (a glucan binding domain (GBD) coupled to the second catalytic domain, CD2 (i.e., GBD-CD2)) that is capable of adding α-(1,2) branching to dextrans.
Various saccharide oligomer compositions have been reported in the art. For example, U.S. Pat. No. 6,486,314 discloses an α-glucan comprising at least 20, up to about 100,000 α-anhydroglucose units, 38-48% of which are 4-linked anhydroglucose units, 17-28% are 6-linked anhydroglucose units, and 7-20% are 4,6-linked anhydroglucose units and/or gluco-oligosaccharides containing at least two 4-linked anhydroglucose units, at least one 6-linked anhydroglucose unit and at least one 4,6-linked anhydroglucose unit. U.S. Patent Appl. Pub. No. 2010-0284972A1 discloses a composition for improving the health of a subject comprising an α-(1,2)-branched α-(1,6) oligodextran. U.S. Patent Appl. Pub. No. 2011-0020496A1 discloses a branched dextrin having a structure wherein glucose or isomaltooligosaccharide is linked to a non-reducing terminal of a dextrin through an α-(1,6) glycosidic bond and having a DE of 10 to 52. U.S. Pat. No. 6,630,586 discloses a branched maltodextrin composition comprising 22-35% (1,6) glycosidic linkages; a reducing sugars content of <20%; a polymolecularity index (Mp/Mn) of <5; and number average molecular weight (Mn) of 4500 g/mol or less. U.S. Pat. No. 7,612,198 discloses soluble, highly branched glucose polymers, having a reducing sugar content of less than 1%, a level of α-(1,6) glycosidic bonds of between 13 and 17% and a molecular weight having a value of between 0.9×105 and 1.5×105 daltons, wherein the soluble highly branched glucose polymers have a branched chain length distribution profile of 70 to 85% of a degree of polymerization (DP) of less than 15, of 10 to 14% of DP of between 15 and 25 and of 8 to 13% of DP greater than 25.
Saccharide oligomers and/or carbohydrate compositions comprising the oligomers have been described as suitable for use as a source of soluble fiber in food applications (U.S. Pat. No. 8,057,840 and U.S. Patent Appl. Pub. Nos. 2010-0047432A1 and 2011-0081474A1). U.S. Patent Appl. Pub. No. 2012-0034366A1 discloses low sugar, fiber-containing carbohydrate compositions which are reported to be suitable for use as substitutes for traditional corn syrups, high fructose corn syrups, and other sweeteners in food products.
There remains a need to develop new soluble α-glucan fiber compositions that are digestion resistant, fermentation resistant by microbiota in the lower gastrointestinal tract, have low viscosity, and are suitable for use in foods and other applications. Preferably the α-glucan fiber compositions can be enzymatically produced from sucrose using enzymes already associated with safe use in humans.